The invention will be described as intended for the production of printing plates primarily for lithographic offset or other types of printing, but is not so limited. The application of the invention to printing plate manufacture is advantageous because of the requirement for printing that there be dots or other geometric shapes formed on the plate which will hold or repel ink as in the technique which is familiarly known as ben day screen printing. Obviously, the production of a visible image through the use of dots or the like which are so fine that they cannot be resolved by the naked eye is advantageous for fields other than printing because the data representing an image can be conveniently stored compactly and called up at will.
The electrophotographic member or film of the invention is basically disclosed in U.S. Pat. No. 4,025,339 which is owned by the assignee of this application. The said U.S. Pat. No. 4,025,339 discloses a photoconductive coating which is sputtered onto different types of substrates and which is invested with certain unique porperties and characteristics to be mentioned hereinafter. The scanning of a master image by a laser or fine light beam and its conversion into binary signals for the production of printing through the use of a moving ink jet is disclosed in U.S. Pat. No. 3,604,846 of which one of the applicants herein is a coinventor.
It is believed that the invention herein is unique in that the ability to "write" information for imaging onto an electrophotographic member at extremely high speed and with a resolution that is many orders better than ink jet printing, for example, arises from an unobvious combination. Known commercially available electrostatic media comprising amorphous selenium members and zinc-oxide-resin paper cannot be imaged at high speeds of the order of microseconds at low energy. The electrophotographic member of the invention, besides having many advantages that are not possessed by available electrophotographic materials, can be imaged in nanoseconds which means that the image can be applied at extremely high speed without sacrificing the high resolution capability of the coating. Furthermore, the member can be imaged by techniques not suitable for use with other electrostatic media.
Considering high energy beams of light, as for example, laser beams, the principal problems which have frustrated their use in attempts to image electrophotographic members have been (a) slow speed of available electrophotographic members, (b) light scatter and the low resolution and (c) the inability of known media to discharge completely. In known electrophotographic media, extremely high energy beams would be required to achieve a degree of discharge which can approach absence of charge representing clean background imaging, even if such were possible.
The commercially available paper member which uses zinc-oxide-resin as a photoconductor and the amorphous selenium drum cannot achieve high speed imaging, crisp images, high resolution, complete discharge. In order to be able to do such imaging, the laser beam used would have to have extremely high energy that is likely to burn the material. It is not known that any success has been achieved with these two media, but even complete success would not provide an electrophotographic member that is flexible, archival in quality, robust and tough in construction and capable of being used directly as a printing plate. Certainly, a transparent printing plate could never be produced with prior art of these electrophotographic media.
The photoconductive coating of the invention has such a high quantum gain that in the visible spectrum for the most part the absorption of a single photon will cause the discharge of approximately more than one electron at the blue end of the spectrum and almost one electron in the green-yellow center so that the energy required of a laser beam which is writing on a charged surface is extremely low. The speed of the member is such that it can respond to charge or discharge in nanoseconds, which is a necessity if the imaging is to be done at high speeds. For example, if a large area of an electrophotographic member is required to be imaged as in the manufacture of newspaper printing plates, the difference between about two or three minutes for the invention and an hour at low energy which would be required to image other media is intolerable. Furthermore, if the imaging of other media additionally produces a resolution equivalent to only a few lines per millimeter as compared to from 8 to 16 lines per millimeter achieved in plates of the invention, the use of laser techniques with electrophotography is not justified. In the case of the invention, a cylinder of the electrophotographic material of the invention 15 inches in circumference is capable of being imaged with a resolution of at the minimum 8 lines per millimeter at the rate of about 1 inch per second. It is clear from known information that the equivalent imaging of a zinc oxide or selenium member would require substantially more time and even if successful the resolution could not be achieved which is capable of being achieved with the invention.
Considering the response of the different media to laser beams or fine pencils of light, in the case of the electrophotographic film of the invention complete discharge of the film can be achieved using energies of the order of microwatts. The same size beam for imaging selenium or zinc-oxide-resin must have energies of the order of milliwatts and even watts in some cases placing important and stringent qualifications on the type of laser that can be used. Zinc oxide electrophotographic paper is formed by actually incorporating finely ground particles of zinc oxide in a matrix of organic resin and this material requires as much energy to image as required for many photographic films. At that, this does not produce a transparent member as can be achieved with photographic film.
It is known to image silver photographic film with laser beams. The thickness of the emulsion in photographic film is of the order of fifty times the thickness of the thin film coating of the electrophotographic members of the invention; hence the laser beam scatters internally in photographic film and prevents the achievement of fine resolution. Photographic film is processed chemically and a good deal of its sensitivity is represented by the manner in which it is processed, but its original speed is related to grain size. The faster the film, the larger the grain size. A compromise must be made between the speed of writing the information onto the film and the eventual size of grain. Laser beams are capable of being modulated by acousto-optical devices at very high speeds. For example, laser beams can be turned on and off in 20 nanoseconds. No known commercial photographic film is capable of being imaged at this speed with the same resolution as the invention, and certainly not with a beam that has energy as low as microwatts.
Another aspect of the imaging of photographic film is concerned with resolution, especially where the tones of an image are dependent upon the fidelity of dots and their placement. The particles of silver which are precipitated in the processing of silver halide film grow in a more or less haphazard manner so that the uniform morphology of dots, for example, projected onto the film is not preserved. This affects the crispness and the resolution of images.
The film or member of the invention has a photoconductive coating which is anisotropic optically in that there is no scatter of light when the laser beam strikes the coating, the incident beam penetrating and discharging in a depth less than half a micron. The crystalline structure has internal reflection and does not scatter normally in incident light. The latent image is formed at or on the surface, and subsequent toning responds only to the sharpness of the surface latent image.
With respect to resolution, mention made of 8 to 16 lines per millimeter is a matter of practicality for the manufacture of a printing plate in a short time. The only limiitation to the resolution capability of the film of the invention is the size of the incident radiant energy beam and the size of the smallest information center in the film. With respect to the former, it is not yet feasible to produce beams of light with lasers and the like that are of the same order as the crystallite size of the electrophotographic film of the invention. With toner particles smaller than this size, which is of the order of 700 Angstroms, the theoretical resolution of the film of the invention would be over 10,000 lines per millimeter.
Practical considerations dictate limits at the present time. Diffraction limited lens systems can produce spot sizes of 0.2 micron but a working lens system to approach the realistic maximum resolution is one in which the beam is about 2 microns in diameter. The problem of achieving this is both optical and mechanical. Even at ten times larger spot size than its present feasible potential, such a beam would be useless for imaging selenium and zinc oxide members.
The laying down of a latent image with high resolution is only one criterion of successful imaging. Another which is of importance is the preservation of the image until the electrophotographic member has been developed by toning. In the intervening time, the latent image is capable of being dissipated or dissolved due to migration of charge. In the case of the electrophotographic member of the invention, the coating is electrically anisotropic which is represented by an extremely high resistivity laterally and a low resistivity when discharge occurs transversely, i.e., through the thickness of the photoconductive coating. The surface resistivity is of the order of 1.times.10.sup.17 ohms per square and higher, it being unlikely that there will be any migration of the charge as there is in the case of selenium and zinc oxide. Selenium owes its usefulness to a very high surface resistivity in darkness, but this is not different from its resistivity transversely so that as soon as an image is formed both resistivities decrease with the result that the charge slowly migrates. There is also a tendency to concentrate the toner where the charge gradient is the greatest, that is, at places where high and low charges exist side by side. Thus, images have a tendency to have their border areas darker than other locations and have the centers of dark areas light instead of solid. This so-called edge effect does not occur in the film of the invention.
Mention made above with respect to total discharge is related to the need for clean backgrounds in the development of images that are composed of dots or other geometric shapes. If there is incomplete discharge as is the case of selenium members, upon toning, the smaller toner particles tend to gather in the areas which are supposed to be colorless and destroy the fidelity of the resulting image. As stated, the member of the invention can be completely discharged so that the background need have no vestigial or remaining charge to attract toner. This is essential where the film is to be used to make a printing plate or, as in the case of the invention, will comprise the printing plate.
The electrophotographic member of the invention has as one of its important attributes the ability to be discharged incrementally so as to achieve varying degrees of surface charge whose potential is proportional to the degree of darkness of the original or master image which was being reproduced. Considerable effort was exerted to utilize this attribute. On this account, it was known that an important factor in achieving an eventual image was the control of toning time to achieve the proper grey scale. Processing was thus considered critical.
According to the invention, grey scale is achieved by the number of dots and for high quality, their placement in a given incremental area with respect to other areas. Each dot is saturated, that is, as black as possible. The processing is not critical. All that is required is to match the toner to the minimum voltage which is achieved during the laser writing. The time required for writing a complete image need only be limited by the voltage chosen as the minimum, this being the dark decay voltage. There are only two types of incremental areas in the latent image, namely those comprising dots whose voltages are above the minimum voltage established for toning and those areas which have no surface potential at all. The dots thus are all saturated and when toned will have an absolute toned value relative to the surround which would normally be dead white (in the case of black and white toning). It has been found that the electrophotographic member of the invention is ideal for this type of imaging since it is capable of being absolutely toned at very low voltages, say of the order of ten volts and less, while also being capable of total discharge in areas alongside of the toned increments.
It will be appreciated that the electrophotographic member of the invention was not originally intended to be used in this manner, i.e., where its superior abilities to exhibit gradients of charge for achieving grey scale are not utilized.